Most polymers or blends of polymers are non-conductive. As such, a static charge build-up may result during processing and use of the polymer. The charged polymer molded parts can attract dust, which are small particles, and can thus interfere with a smooth surface appearance. The attracted particles to the surface of a molded article may also cause a decrease in the transparency of the article. In addition, the electrostatic charge can be a serious obstacle in the production process of such polymers.
Anti-static agents are small molecule materials which can be added to polymers to reduce their tendency to acquire an electrostatic charge, or when a charge is present, these anti-static agents promote the dissipation of such a charge. The anti-static agents are usually hydrophilic or ionic in nature. When present on the surface of polymeric materials, they facilitate the transfer of electrons and thus eliminate the build up of a static charge. Anti-static agents have been applied in two ways. One method uses external anti-static agents that are applied by spraying the surface or dipping of the polymeric material. The second method uses internal anti-static agents, which are added to the polymer before processing. It is necessary for anti-static agents applied in this manner to be thermally stable and able to migrate to the surface during processing.
Since there are many anti-static agents having surface-active additives as their main constituent, appropriate ones may be selected therefrom according to the situation. Surface activity can involve both migration of the agent to the surface of the article during molding as well as their ability to function to eliminate the build up of a static charge at the article surface. When used as an internally-applied anti-static agent, however, anionic surface active additives are difficult to handle because they are inferior in compatibility and uniform dispersibility and tend to decompose or deteriorate when heated. Cationic surface-active additives containing qualernary nitrogen in their molecules and amphoteric surface-active additives, on the other hand, can be used only in limited situations because they are extremely poor in heat resistance, although their anti-static characteristics are good. As for non-ionic surface-active additives, they are more compatible with polymeric materials, but tend to be weak in anti-static characteristics and their effects disappear with time at normal or high temperatures. Moreover, because of the limited thermal stability of these non-ionic surface-active anti-static agents, their use with engineering thermoplastic resins, such as aromatic polycarbonates, is also limited due to the temperatures at which such resins are processed. Thus, these types of surface-active additives adversely affect the optical properties of aromatic polycarbonates.
Although metal salts of organic sulfonic acids have been reported, especially as internally applied anti-static agents for polycarbonates and polyester resins which are molded at high temperatures, they are not sufficient in compatibility with resins, nor are they heat resistant. An adverse consequence of insufficient compatibility is that transparency characteristics of certain polymeric materials are lost with such anti-static agents. Also onium salts (such as phosphonium and ammonium salts of organic sulfonic acids) are known, particularly ammonium and phosphonium salts of perfluorinated alkyl sulfonates.
While there are known anti-static agents presently available, there remains a need for further improvements to provide anti-static agents that provide a thermoplastic composition with good anti-static properties, and good retention of the anti-static properties over time and/or after exposure to water. Preferably, such improvements are provided without detrimentally affecting the optical properties of the thermoplastic, including transparency.